Field of the Invention
The present disclosure relates to a method for processing a silicon substrate.
Description of the Related Art
A method by using reactive ion etching, which is one type of dry etching, is mentioned as a method for forming a through hole in a silicon substrate. The processing of the silicon substrate by using the reactive ion etching is a method in which the through hole is formed in the silicon substrate by using an etching gas. In particular, in the case where a liquid supply port serving as a through hole is formed in the silicon substrate used for a liquid ejection head typified by an inkjet head, the reactive ion etching can be used. According to the reactive ion etching, a hole having a vertical shape is formed easily, so that the through hole does not widen laterally easily and the size of the silicon substrate is decreased.
Two methods, single-sided processing and double-sided processing, are mentioned as processing methods to form the through hole in the silicon substrate by reactive ion etching. One surface of the silicon substrate is specified to be a first surface and the other surface is specified to be a second surface which is a surface opposite to the first surface. At this time, in the case of the single-sided processing, etching is started from the first surface of the silicon substrate, and the etching is continued until the second surface of the silicon substrate is reached, so that the silicon substrate is penetrated. On the other hand, in the case of the double-sided processing, etching is started from, for example, the first surface of the silicon substrate, and the etching is stopped in midstream. Consequently, a non-penetrated hole extending from the first surface toward the second surface side is formed in the silicon substrate. Subsequently, etching is performed from the second surface on the opposite side toward the first surface side, and etching is allowed to reach the non-penetrated hole. As a result, the silicon substrate is penetrated. According to the double-sided processing, the shape of the through hole is controlled easily and even a through hole having a complicated shape, for example, a shape in which the width is changed at some midpoint, is formed easily.
In the case where the silicon substrate is processed by the reactive ion etching, a phenomenon called over etching may occur. In the double-sided processing, when the reactive ion etching is performed from the second surface and the silicon substrate is penetrated, as described above, there is no target of etching in the etching direction (the extension direction of hole) and a hole is formed in the direction perpendicular to the extension direction of the hole. This is the over etching and the opening on the first surface side of the through hole is widened as compared with the predetermined shape. This state will be specifically described with reference to FIGS. 7A to 7C. FIGS. 7A to 7C show the state of forming a liquid supply port serving as a through hole 11 in a silicon substrate 1. As shown in FIG. 7A, a silicon substrate 1 having a first surface 2 and a second surface 3 which is a surface opposite to the first surface is provided. The silicon substrate 1 has an etching mask 9 on the first surface 2 side. As shown in FIG. 7B, reactive ion etching is performed from the first surface 2 of the silicon substrate 1 through the opening of the etching mask 9. In this manner, a non-penetrated hole 6 is formed in the silicon substrate 1. Then, an etching mask is also formed on the second surface 3 side and reactive ion etching is performed from the second surface 3 side. When etching is allowed to reach the non-penetrated hole 6 and the silicon substrate 1 is penetrated, a through hole 11 is formed as shown in FIG. 7C. If the etching is further continued, a hole 14 which has been widened laterally by over etching is formed on the first surface 2 side of the silicon substrate 1. That is, the opening on the first surface 2 side of the through hole 11 is widened laterally. In the case of the liquid ejection head, the hole formed by the over etching may be widened to a region provided with an energy-generating element or a wiring thereof and the reliability of the liquid ejection head may be degraded. Also, in the case where an object having low electrical conductivity (it may be an insulator) is present in the etching direction when the reactive ion etching is performed, a phenomenon called notching may occur. The notching is also one type of over etching. In the case where the reactive ion etching is performed, usually, an object having low electrical conductivity is present in the etching direction and, therefore, the notching is also one of issues.
U.S. Pat. No. 7,481,943 discloses that a non-penetrated hole is formed from a first surface of a silicon substrate by reactive ion etching and, thereafter, a resin is filled into the non-penetrated hole. In this method, after the resin is filled into the non-penetrated hole, reactive ion etching is performed from a second surface, the etching is allowed to reach the resin, and the resin is removed finally, so that a through hole is formed.
It is considered that according to the method described in U.S. Pat. No. 7,481,943, the position of occurrence of over etching is shifted from the front surface of the substrate to the back surface side and, as a result, lateral widening of the through hole is suppressed.